Fix ticket #3057: apply HTML escaping on special characters in Twitter import

Fix ticket #3057: apply HTML escaping on special characters in Twitter import

Changes the replacement of Twitter "entities" from in-place reverse ordering ('to preserve indices') to a forward-facing append-in-chunks that pulls in both the text and link portions, and escapes them all.
This unfortunately means first *de*-escaping the < and > that Twitter helpfully adds for us.... and any literal &blah;s that get written. This seems to match Twitter's web UI, however horrid it is.